Power supplies and associated systems typically have voltages critical to proper equipment operation, each of which voltages must be monitored and maintained in accordance with an acceptable voltage range of voltage window. One particular technique for voltage monitoring involves the use of separate comparators, each receiving a separate input and a reference derived from a common threshold voltage or set of respective voltage thresholds. However, the use of multiple comparators requires additional summing logic or combination elements, as well as matching of the components of each comparator if identical performance relative to the input signals is desired. The resulting comparator circuit to accommodate multiple-input voltages is large, complex, and expensive.
Another known circuit comprises a single integrated circuit comparator which incorporates a differential input stage, with additional input transistors connected in parallel (to the collector and emitter of the signal input transistor). Each parallel input transistor base receives a respective input voltage to be compared. The other (differential) transistor in the differential pair receives a reference voltage to establish a threshold. However, this approach provides true sensing of only one input voltage at a time. Should more than one input signal approach the threshold voltage, the respective parallel-connected input transistors each begin to partially conduct. As a result, the total current for all of the partially conducting transistors will equal the current for a single input transistor having a respective input signal exactly at the threshold voltage. Therefore, the result is to effectively lower the reference voltage threshold when a plurality of input signals simultaneously approach the threshold voltage, providing erroneous indication of the status of the input signals.
In particular, where K represents the number of parallel-input transistors having respective sense inputs, the point at which the differential amplifier output signal changes to indicate that a signal has crossed the reference threshold, will be lowered by a voltage equal to 26 m V.times.1n(K) at room temperature, or 35 mV .times.1n(K) at 125.degree. C., where 1n denotes a natural logarithm. For four inputs (K=4) at room temperature, this is equivalent to a 36 mV threshold change, representing a significant error in the voltage sensing operation of the comparator.